This study aims to determine the effects of protease inhibitor (PI) therapy on metabolism and body composition in normal, healthy seronegative controls, in order to determine whether adverse effects associated with PI therapy in patients with HIV/AIDS are related to viral suppression or are an intrinsic effect of PI drugs per se. A 12 week study of Crixivan (a PI therapy) in men and women will be carried out, with metabolic infusion studies at baseline and 12 weeks of therapy, in addition body composition, energy expenditure , fuel selection and endocrine function will be evaluated. A six week out-patient assessment will also be carried out.